The present invention deals with the formation of substrates used in magnetic media. More particularly, the present invention uses spectral signature analysis for monitoring a process of forming substrates used in magnetic discs.
A magnetic disc, such as one used in a computer disc drive, is a flat circular platter with a magnetic surface on which data can be stored by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values. Typically, the magnetically polarized portions are arranged in a plurality of radially concentric tracks on the disc surface to aid in location and read back of the data.
A magnetic transducer moving relative to the magnetic disc along a given track generates an electrical signal (the "read signal"), which is related to the states of polarization encountered along the track. The magnetic transducer is mounted on a read/write head which has an air bearing for creating a hydrodynamic lifting force during disc rotation. This causes the read/write head to fly 0.30 microns (12 microinches) or less above the surface of the magnetic disc.
The disc itself is composed of several layers of material. A substrate, typically an aluminum alloy, is the supporting base upon which thin films of magnetic layers and overcoat layers are deposited. The magnetic disc retains certain surface characteristics of the substrate even after deposition of the other layers. These characteristics play a critical role in supporting the air bearing on the read/write head in flight above the disc surface.
Substrates are typically produced by a formation process having several process steps. Each process step is performed according to a number of process parameters. To maintain a proper head/disc separation during drive operation, each substrate produced must adhere to certain specifications. Part of the specification requires the substrate surface to be within certain limits described in terms of axial runout, velocity and acceleration. These characteristics are collectively known in the disc drive industry as RVA, and they can vary along with variations in the process parameters used in forming the substrate.
In traditional production testing of substrate RVA, the substrate is mounted on an air spindle rotating at approximately 60 Hertz (3600 rpm). A capacitance probe moves from the outer radius to the inner radius of the substrate forming a spiral pattern on the surface of the substrate. An axial runout signal representing axial runout along the substrate surface is generated by the capacitance probe and processed by analog low-pass filters and op-amp differentiators. This signal processing provides velocity and acceleration signals. These signals are provided to peak detect circuits which are used in go/no-go type substrate screening.
Such tests can only measure a product's specification conformance. They lack any capability in process monitoring. Therefore, where substrates fail the go/no-go type screening process, these tests cannot be used to determine what portion of the substrate formation process is causing the substrates to be out of specification. Further, these tests cannot be used to experiment with, and improve the substrate manufacturing process or to adjust the process parameters.